A tire pressure monitoring system (TPMS) is an electronic system that monitors the air pressure of tires on a vehicle. Such systems generate a pressure signal using an electromagnetic (EM) radio frequency (RF) signal, which is transmitted to a receiver and corresponds to a pressure within the tire. When the tire pressure drops below a threshold pressure, an indicator is used to signal the vehicle operator of the low pressure. The vehicle operator can remedy the problem by filling the low tire to increase the pressure therein. A direct tire pressure monitoring system (TPMS) uses pressure sensors that are directly mounted to the wheels or tires of the vehicle.
Direct TPMS typically relies on a cold inflation pressure threshold, which is the inflation pressure of the tires before the car is driven and the tires warm-up, and is typically a setting recommended in the vehicle owner's manual. Recently however, some markets require that under-inflation warning thresholds be based upon in-service tire pressure, also known as warm placard tire pressure. Tire pressures recommendations are provided by motor vehicle manufacturers in the form of placards. In-service tire pressure is referred to as warm placard pressure and is defined as the tire pressure after the tire temperature has reached a steady-state condition. This steady-state condition typically occurs after the vehicle has been traveling above a threshold speed for a predetermined amount of time, typically twenty minutes of driving.
Determining the warm placard pressure can be difficult because it requires knowledge of an original pressure set-point intended by the vehicle operator. Furthermore, ambient temperature influence on tire pressure may create variations in warm placard pressure within a given day and day to day. When ambient temperature decreases, tire pressure also decreases. When the tire is driven, the temperature increases, as does the tire pressure.
Other factors may also affect the threshold value for warm placard tire pressure. For example, vehicles in some markets also have additional variations in recommended pressure, such as pressure recommendations that are split between the front and rear tire locations, also known as split-placard or multiple placard pressures. Other variations may include recommendations for full vs. lightly loaded conditions and vehicle speed. In order to monitor such additional variations, the direct TPMS must determine the location of each sensor, which is typically accomplished by way of expensive and complex hardware.
There is a need for determining warm placard pressure for direct TPMS and in particular determining warm placard pressure for multiple placard pressure requirements.